This invention relates to an improved screw for use in an extruder for working a wide range of solid plastic materials into a substantially homogeneous, molten state suitable for formation into any desired shape by extrusion or injection into a die or mold. More particularly, the improved screw of the present invention is most readily used in what is known as a single screw extruder.
Extrusion, injection molding or blow molding with a single screw extruder, for example, having a screw of conventional design shown in FIG. 1, includes feeding the solid polymeric or plastic material in pellet, chip, powder, or flake form to the feed end of the extruder through a hopper mounted on an opening of the heated barrel in which a screw is rotatably mounted. The screw has at least one helical thread with a minimum clearance to the barrel, integrally mounted of formed on the core to create a helical channel, down which the plastic material is moved downstream from the feed end to the discharge end by forces exerted by the rotation of the screw. The solid plastic material fed into the screw channel is compacted into a solid plug or solid bed and the solid bed melts as it travels down the screw channel. The molten plastic material is collected by the wiping action of the thread into a melt pool. The melt pool gradually increases as the solid bed gradually melts, eventually occupying the entire screw channel.
Molten plastic materials have a very high viscosity and a large amount of heat is generated in the melt pool due to shearing of the melt pool by the rotation of the screw. Thus, the melt pool becomes hotter as it travels down the screw channel and often becomes undesirably hot by the time it reaches the discharge end. Heat transfer from the melt pool to the solid bed is inefficient because of the low thermal conductivity of plastic materials and the limited contact area between the melt pool and the solid bed. Increased heat transfer from the hot, molten plastic material in the melt pool to the cold, solid plastic material in the solid bed is highly desirable in order to reduce the temperature of the molten plastic material discharged from the extruder, increase melting capacity of the extruder and the increase energy efficiency of the extrustion process.
R. A. Barr and C. I. Chung, in U.S. Pat. No. 3,487,503, issued Jan. 6, 1970, use a plurality of pegs mounted crosswise in the screw channel suffiently near the discharge end whereon only a small fraction of the plastic material remains as solid in the screw channel. The pegs promote mixing of the solid plastic material with the molten plastic material bh breaking up the residual solid bed and thus increase the contact area between the solid plastic material and the molten plastic material, resulting in increased heat transfer from the molten plastic material to the solid plastic material.
H. Schippers et al, in U.S. Pat. No. 3,701,512 issued Oct. 31, 1972, divided the screw channel near the discharge end into a pair of side-by-side sub-channels of equal width by a second thread. The diameter of the second thread is sufficiently smaller than the diameter of the barrel such that its clearance to the barrel allows the plastic material to flow over the second thread. The depths of the two side-by-side sub-channels vary continuously and oppositely along the length of the passages so that the combined passage cross-sectional area of the two sub-channels is maintained constant. As one sub-channel becomes shallow in depth with diminishing cross-sectional area, the other sub-channel becomes depper correspondingly with enlarging cross-sectional area, so that the plastic material is forced to move from the diminishing sub-channel into the enlarging sub-channel flowing over the second thread. The second thread gives shearing to the plastic material while flowing over it. Such mechanism of moving the plastic material from one sub-channel into the other sub-channel is repeated a number of times. It is noted that, in the screw of H. Schippers et al, the first thread with a minimum clearance to the barrel continues to remain as the wiping thread throughout the entire length of the screw while the second thread with a large clearance to the barrel remains for its entire length as the shearing thread over which the plastic material can flow. Such structure of the screw forces the plastic material to move from one sub-channel located downstream of the second thread or toward the discharge end, to the other sub-channel upstream of the second thread or toward the feed end of the second thread in one instance and then forces the plastic material to move back from the sub-channel located upstream of the second thread to the sub-channel located downstream of the second thread in the following instance. The rotation of the screw aids the movement of the plastic material from the sub-channel located downstream of the second thread into the sub-channel located upstream of the second thread, but resists the movement of the plastic material from the sub-channel located upstream of the second thread into the sub-channel located downstream of the second thread. The rotation of the screw causes the plastic material to move, relative to the screw, in the direction opposite to the screw rotation, and thus any movement of the plastic material in the direction of the screw rotation such as the movement of the plastic material from the sub-channel upstream of the second thread into the sub-channel downstream of the second thread is undesirable and can undermine the performance of the screw.
G. A. Kruder, in U.S. Pat. No. 4,173,417 issued Nov. 6, 1979, discloses a screw similar to that of H. Schippers et al. Kruder also divides the screw channel into two sub-channels of equal width by a second thread. The depths of the sub-channel are cyclically varied with crests and valleys such that the melting of the solid plastic material is maximized over the crests and the clearance of the second thread to barrel is made such that only molten plastic material can flow through the clearance while restricting the flow of solid plastic material through the clearance. Kruder states that the provision of the sub-channels, whose combined passage cross-sectional area is not constant along the length of the passages, serves to maximize the complexity of the flow pattern of the plastic material. It is noted that Kruder also uses the first thread with a minimum clearance to the barrel as the wiping thread throughout the entire length of the screw and the second thread with a large clearance as the shearing thread for its entire length.
Therefore, the screw of Kruder also suffers from the same disadvantage of moving plastic material in the direction opposite to the natural flow caused by the rotation of the screw as discussed for the screw of Schippers et al.
It is an objective of this invention to obtain an efficient mixing of the solid plastic material with the molten plastic material inside a single screw extruder in order to increase heat transfer from the hot, molten plastic material to the cold, solid plastic material thereby achieving increased melting capacity of the extruder, a lower temperature of the molten plastic material discharged from the extruder and a higher energy efficiency of the extrusion process.
A further objective is to achieve an efficient mixing of the solid plastic material by making the plastic material, both solid and molten, to flow only in the direction of the natural flow caused by the rotation of the screw.
Objectives ancillary to the foregoing objectives are to teach and provide an extruder screw to accomplish said objectives.